In a common rail fuel injection system conventionally known as a fuel injection system for a diesel engine, a high pressure fuel is accumulated in a common rail, and the high pressure fuel accumulated in the common rail is injected into combustion chambers of respective cylinders of the engine, through multiple injectors provided at the respective cylinders of the engine, at a predetermined timing. The high pressure fuel is supplied into the common rail from a fuel supply pump through a high pressure fuel pipe, wherein the fuel supply pump pressurizes fuel fed into a pressurizing chamber through an electromagnetic valve, because it is necessary to always accumulate in the common rail the high pressure fuel corresponding to a fuel injection pressure.
A fuel discharge amount discharged from the fuel supply pump is controlled by adjusting an opening area of a fuel suction passage communicating the feed pump with the pressurizing chamber through a suction valve, wherein fuel suction amount of the fuel flowing from the feed pump into the pressurizing chamber is adjusted by a pump driving current to a solenoid coil of the electromagnetic valve. An electromagnetic valve for the above purpose is proposed (for example, Japanese Patent Publication No. 2002-106740), in which an electromagnetic type suction fuel amount adjusting valve comprises; a spool valve having a function of a valve body for adjusting an opening area of fluid passage by moving in a valve sliding space and having a function of an armature for forming a magnetic circuit; and a valve casing having a function of a cylinder for movably accommodating the spool valve and having a function of a stator for forming the magnetic circuit.
In the electromagnetic type suction fuel amount adjusting valve disclosed in the above Japanese Patent publication, however, a centering of the spool valve in the valve sliding space of the valve casing can not be easily achieved due to a difference in a magnetic force, which is caused by a deviation of a clearance formed between an outer peripheral surface of a sliding portion of the spool valve and an inner surface of the valve sliding space of the valve casing. Then, the outer peripheral surface of the sliding portion of the spool valve is pressed against the inner surface of the valve sliding space of the valve casing, and thereby a lubricating performance is deteriorated between the outer peripheral surface of the sliding portion of the spool valve and the inner surface of the valve sliding space of the valve casing. As a result, the sliding portion of the spool valve may be stuck to the inner surface of the valve sliding space, and it may cause a problem of a decrease of durability. In the above method, in which the spool valve is moved relative to the fluid port of the valve casing to adjust the fuel flow amount, since the spool valve is slowly moved to control the opening area of the fluid passage of the fluid port, a sliding speed of the spool valve with respect to the fluid port of the valve casing is extremely low, it would become difficult to form an oil film between the outer peripheral surface of the sliding portion of the spool valve and the inner surface of the valve sliding space of the valve casing. The lubricating performance is thereby further deteriorated.
Accordingly, the sliding portion of the spool valve can not be smoothly moved with respect to the fluid port of the valve casing, when the pump driving current is applied to the solenoid coil of the electromagnetic type fuel suction amount adjusting valve and the armature of the spool valve is attracted toward an attracting portion of the stator of the valve casing. For example, an adjusting performance for fuel flow amount is deteriorated, as a result that response of the electromagnetic valve is deteriorated with respect to a pedal stroke of an acceleration pedal operated by a vehicle driver. Accordingly, a fuel suction amount of the fuel flowing from the feed pump into the pressurizing chamber may not quickly reach at a target amount, a time period during which the fuel amount discharged from the fuel supply pump reaches at a target amount would become longer, and a fuel pressure in the common rail can not quickly reach at a target fuel pressure as the vehicle driver desired. As a consequence, a delay of increasing an engine rotational speed may occur due to a delay of increase in the fuel injection amount, with respect to a change of the pedal stroke of the acceleration pedal operated by the vehicle driver. As above, a problem has occurred, in which an engine performance, such as an acceleration response and so on, is decreased.
In another prior art flow control valve for a common rail fuel injection system, for example, as discloses in Japanese (PCT) Patent Publication No. 2005-530568, the flow control valve adjusts an opening area of a fuel suction passage communicating a low pressure pump (a feed pump) with a high pressure pump (a supply pump), by controlling a magnetomotive force at a solenoid coil and thereby a relative position of a spool valve to a valve casing.
In the flow control valve disclosed in the above Japanese (PCT) Patent Publication, however, a lower side of an outer surface of the spool valve is always kept in contact with the valve casing due to gravity, and a partial wear may be caused because the spool valve is moved to slide in the valve casing with such a condition. Furthermore, a gap between the spool valve and the valve casing becomes smaller at a surrounding area of the contact portion between the spool valve and the valve casing, and thereby a larger magnetomotive force of the solenoid coil is applied to increase the contacting force, so that the partial wear is facilitated.
According to microscopic observation, streaked vestiges of wear are formed in the spool valve and the valve casing in a sliding direction of the spool valve, due to the wear. A contacting area is further increased between the spool valve and the valve casing, and frictional force is increased. This frictional force breaks a balance between the spring force and the magnetomotive force of the solenoid coil, to thereby cause a bad operation (a bad sliding movement) of the spool valve, to deteriorate controllability of fuel pressure in the common rail, and to cause a variation of fuel injection amount. Furthermore, it may increase harmful exhaust gas, deteriorate drivability, increase engine noise, and so on.
The electromagnetic valve disclosed in Japanese Patent Publication No. 2002-106740, as described above, comprises the valve casing having the valve sliding space straightly extending in the axial direction, and the cylindrical spool valve movable in the valve sliding space of the valve casing in the axial direction to control a communication condition between an outlet port and a communication port. A through-hole penetrates through an inside of the spool valve. A spring space is formed, to accommodate a spring, in the valve casing at one side of the spool valve in the axial direction for biasing the spool valve in its valve closing direction. The spring space is communicated with an inlet port of the valve casing through the through-hole.
In the above electromagnetic fuel suction amount control valve, however, fuel is supplied into the through-hole of the spool valve from the inlet port of the valve casing, and the fuel supplied into the outlet port of the valve casing through a flow amount adjusting groove. And the fuel in the spring space mostly stays in the spring space. When a foreign material flowing into the electromagnetic fuel suction amount control valve together with the fuel, or wear-out powder generated by sliding movement between a sliding surface of the spool valve and a sliding surface of the valve casing flows into the spring space, they stay in the spring space. Therefore, it may cause a problem that the foreign material or the wear-out powder may not easily flow out from the spring space.
A flow amount control portion is formed to control fuel flow amount by changing the opening area of the flow passage of the outlet port, when the spool valve is moved in its axial direction within the valve sliding space of the valve casing. For that purpose, a minimum gap, which is necessary in order that the spool valve is moved in its axial direction within the valve sliding space of the valve casing, is formed between the sliding surface of the spool valve and the sliding surface of the valve casing. Then, the foreign materials or the wear-out powder staying in the spring space may flow into the gap along with the axial movement of the spool valve, to break into the gap between the sliding surface of the spool valve and the sliding surface of the valve casing. A sliding resistance of the spool valve with respect to the sliding surface of the valve casing may be increased. As a result, a sliding performance of the spool valve is deteriorated and may cause a problem of a bad sliding movement of the spool valve. In the case that such bad sliding movement has occurred, a control characteristic of a pump discharge amount is deteriorated with respect to a value of a pump driving current to the solenoid coil of the flow control valve, and finally it may cause a problem of a decrease of an engine output.